1. Field of the Invention
The present invention relates to a lamp suitable for the application to lamp of vehicles such as automobiles, and more particularly to a lamp in which a lamp cover is laser-beam welded to a lamp housing and a method and apparatus for fabricating the lamp.
2. Related Art
As lamps for vehicles, there have been widely proposed lamps in which a lamp body, as a lamp housing, and a lens, as a lamp cover for covering a front opening in the lamp housing so as to seal the interior of the lamp body, are separately produced from resin. Then, the lens is welded integrally to the lamp body. Conventionally thermal welding has been used in welding. These components together. In recent years, a so-called laser-beam welding technique has been put into practical use in which resin materials are fusion welded together by making use of laser beam energy. FIG. 1 is an exploded perspective view of an example of a lamp which is fabricated by means of the laser-beam welding, in which a flange portion 11 is formed around a front opening in a lamp body 1, and a lens leg portion 21 provided around a circumferential edge of a lens 2 is laser-beam welded to the flange portion 11.
FIG. 2A is a partially-cutaway front view of the lamp LAMP, and FIG. 2B is an enlarged view of a portion P of the lamp shown in FIG. 1, in which abutment surfaces of the flange portion 11 and the lens leg portion 21 are made integral with each other via laser-beam welding X. As shown in a conceptual drawing of FIG. 8A, the lens leg portion 21 around the circumferential edge of the lens 21 made of a transparent resin which is placed over the front opening in the lamp body 1 made of a chromatic colored resin is brought into abutment with the flange portion 11 provided around the lamp body 1 at an end face thereof, and a laser beam LB is transmitted through the lens 2 so as to be emitted to a surfaces abutment portion S made up of the abutment surfaces of the two components. The flange portion 11 of the lamp body 1 is heated through the emission of the laser beam LB. The heat generated is transmitted to the lens leg portion 21 via the surfaces abutment portion S, whereby the flange portion 11 and the lens leg portion 21 are each fused at the surfaces abutment portion S, so that both the flange portion 11 and the lens leg portion 21 are joined together through the laser-beam welding X. This laser-beam welding technique is described in, for example, JP-A-2001-243811.
In this type of lamp, normally, the flange portion is formed in a larger size than that of the lens leg portion because in the event that a circumferential surface of the lens protrudes outwardly of the lamp body when the lens is fusion welded to the lamp body, the lens tends to easily chip at the end face thereof and the external appearance of the lamp is damaged, which is not preferable. Due to this, as shown in FIG. 8B, when the lens leg portion 21 is brought into abutment with the flange portion 11, part of the flange portion 11 is exposed outwardly from the lens leg portion 21. When the laser beam LB is emitted to the surfaces abutment portion S of the flange portion 11 and the lens leg portion 21, there may be a chance that part of the laser beam LB is emitted to the exposed portion S′ of the flange portion 11, which is not in abutment with the lens leg portion 21. In this exposed portion S′, it is difficult for the heat generated in the flange portion 11 to be transmitted to the lens leg portion 21. Due to this, heat is accumulated in the exposed portion S′ of the flange portion 11 to thereby produce an excessively heated state, whereby the surface of the exposed portion S′ gets burned to produce a burn Y thereon. This causes problems in that the quality of the texture of the resin on the surface of the flange portion 11 is modified due to the burn Y. Thus, the welding strength to the lens leg portion 21 is affected and thereby decreased. Also, the black burn Y is exposed on the flange portion 11 of the chromatic colored lamp body to thereby damage the external appearance of the lamp LAMP.
To cope with these problems, it has been attempted to control the converging conditions of the laser beam and the position of the optical axis thereof may be controlled such that no laser beam is emitted to the exposed portion on the surface of the flange portion when emitting the laser beam. However, due to the beam diameter of the laser beam being small and the light emitted to the lens leg portion from the outside being refracted and scattering within the lens leg portion before it is emitted to the flange portion, it is extremely difficult to control the emitting range of the laser beam with high accuracy, and it is nearly impossible to solve the above problems. In addition, even in the event that accuracy at which the laser beam is emitted to a required position is increased, since it is difficult to avoid a variation in assembling of lenses with bodies during mass production, it is difficult to prevent the emission of the laser beam to the exposed portion on the surface of the flange portion. For example, FIG. 9A shows that the flange portion 11 is made identical to the lens leg portion 21 in size. However, when the laser beam LB is emitted to a portion where the circumferential end surface of the flange portion 11 is exposed, a burn Y is produced thereon. In addition, as shown in FIG. 9B, even in the event that the radial length of the flange portion 11 is shorter than that of the lens leg portion 21 and hence the circumferential end surface of the flange portion 11 resides radially inwardly of the circumferential surface of the lens leg portion 21, burns Y are produced on the circumferential end surface and other locations on the lamp body to which the laser beam is emitted.